Process for estimating a parasite phase shift during reception of a multi-carrier signal and the corresponding receiver

ABSTRACT

A method for estimating an interference phase shift when receiving a multicarrier signal formed of a time series of symbols modulating a plurality of carrier frequencies, at least some of the carrier frequencies of at least some of the symbols bearing reference elements, whereof the value at transmission level is known by the receiver carrying out the reception. The phase variation deltan between at least two symbols bearing reference elements is estimated by analysing the reference elements. Each of the reference elements in the estimation is weighted by information representing the noise affecting the carrier frequency bearing the reference element.

The field of the invention relates to the reception of multi-carriersignals. More precisely, the invention relates to estimating the phaseshift introduced by demodulation operations.

The invention is applicable to all types of signals using severalcarrier frequencies, in other words systems using signals transmittedusing the Frequency Division Multiplex (FDM) technique, for example theCoded Orthogonal Frequency Division Multiplex (COFDM) system, usedparticularly within the framework of the European Eureka 147 “DAB”(Digital Audio Broadcasting) and RACE dTTb (digital Terrestrial TVbroadcasting) projects.

In this type of transmission system, the source data to be transmittedare organized in symbols (composed of one or several source data) eachmodulating a carrier frequency chosen among several carriers, during apredetermined time interval. The signal formed by the set of modulatedcarriers is transmitted to one or several receivers that receive anemitted signal disturbed by the transmission channel.

In principle, demodulation usually consists of estimating the responseof the transmission channel for each carrier and for each symbol, andthen dividing the received signal by this estimate to obtain an estimateof the emitted symbol.

A number of demodulation techniques are known. The demodulation may bedifferential or coherent. For example, patent FR-94 07984 deposited bythe same applicants describes one technique facilitating coherentdemodulation using reference symbols (or “pilots”) known in receiversand inserted regularly among the useful symbols.

Therefore, the multi-carrier digital signal considered includes a numberof “pilots”, in other words carriers modulated by known values of thereceiver. Let C be the set of indices of these carriers. If K belongs toC, the complex value modulating carrier number k for the duration ofsymbol number n is denoted P_(n,k). Note that this value is known to thereceiver.

In reception, the value observed on the n^(th) symbol of carrier numberk is denoted R_(n,k). Typically:

R _(n,k) =P _(n,k) H _(n,k) e ^(jφ) _(n)(+noise)  (1)

where H_(n,k) is the complex frequency response of the channel at thefrequency of carrier number k;

φ_(n) is a phase introduced by the demodulation that is to be estimated(φ_(n) is common to all carriers).

H_(n,k) varies slowly with time (H_(n,k) is not very different fromH_(n+1,k)), whereas φ_(n) can vary considerably from one symbol to thenext if the local oscillator in the receiver is badly adjusted.Therefore, estimating φ_(n) is useful to determine the frequency errorof the local oscillator so that it can be corrected afterwards(Automatic Frequency Control AFC).

Furthermore, as described in the documents in French patents FR-95 10067and FR-95 10068 deposited by the applicants who also deposited thispatent application, knowledge of φ_(n) is useful to reduce the biasintroduced by white frequency distortion, and take account of anestimate of this type of white distortion for demodulation.

In practice, only the variations of φ_(n) need to be estimated: thevalue of φ_(o) fixed by definition may be incorporated into the responseof channel H_(n,k). Since H_(n,k) is not very different from H_(n+1,k),(φ_(n)−φ_(n−1)), denoted δ_(n), is conventionally estimated from R_(n,k)using the following formula: $\begin{matrix}{\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}} & (2)\end{matrix}$

This known approach is explained as follows. The modulus of the squareof H_(n,k) will be denoted A_(n,k), and the modulus of the square ofP_(n,k) will be denoted Q_(n,k). According to equation (1),R_(n−1,k)P*_(n,k) gives an estimate of Q_(n,k)H_(n,k)e^(jφn−1).Similarly, R_(n,k)P*_(n−1,k) gives an estimate of Q_(n−1,k)H_(n−1,k)^(ejφn−1). Finally, since H_(n,k) is not very different from H_(n−1,k),H_(n,k)H*_(n−1,k) is close to A_(n,k). Therefore$\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {Q_{n,k}Q_{{n - 1},k}A_{n,k}^{j{({\varphi_{n} - \varphi_{n - 1}})}}} \right\rbrack} \right\}}$

It is useful to weight the terms in the sum by Q_(n,k).Q_(n−1,k).A_(n,k)since it reflects the reliability of each term (the phase is morereliable for larger magnitudes). Therefore, this type of estimate isparticularly suitable for channels with selective frequency, for whichA_(n,k) varies strongly with k.

Finally, the phase δ_(n) of the sum gives an estimate of (φ_(n)−φ_(n−1))for which the reliability increases with the number of carriers of C.

In practice, values of P_(n,k) usually have the same modulus Q. P_(n,k)may be completely independent of n (continuous carriers); in this case,the sum (2) may be simplified since P*_(n,k)P_(n−1,k) is always equal toQ, as follows: $\begin{matrix}{\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {R_{n,k} \cdot R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right\rbrack} \right\}}} & \text{(2a)}\end{matrix}$

In the following, the form of equation (2) will be maintained in orderto remain general, but the form (2a) is obviously preferable in the caseof continuous carriers.

In some cases, an attempt is made to estimate (φ_(n)−φ_(n−t)), wheret>1, rather than (φ_(n)−φ_(n−1)); obviously, the method remains thesame. All that is necessary is to replace subscript n−1 by n−t.

The inventors have observed that although this conventional techniquemay be efficient under transmission conditions in which there is notmuch interference, it is poor in some conditions in the presence ofinterference sources. In this case, some carriers are continuouslypolluted by very strong noise, sometimes much stronger than the usefulsignal. The result of this is some completely distorted terms, usuallywith a strong amplitude, in sum (2). Since δ_(n) is fairly small,particularly when the local oscillator tends towards the rightfrequency), these few terms can completely disturb the result of thesum.

One particular purpose of this invention is to overcome thisdisadvantage with the state-of-the-art.

More precisely, one purpose of the invention is to provide a techniquefor strongly attenuating the effect of interference sources on theestimate of the phase shift induced by the receiver.

In other words, the purpose of the invention is to provide a process anda corresponding receiver that optimizes the estimate of the phase shift,particularly when the phase shift is small.

Another purpose of the invention is to supply a corresponding techniquethat is simple to use and that does not require complex calculations orspecific means in the receivers.

Another purpose of the invention is to provide a similar technique thatdoes not require any modification to the signal to be emitted, and whichis therefore compatible with transmission techniques already used.

These purposes, and others that will become clear in the followingdescription, are achieved according to the invention using a process forestimating a parasite phase shift on reception of a multi-carrier signalformed by a time sequence of symbols modulating a plurality of carrierfrequencies, at least some of the said carrier frequencies for at leastsome of the said symbols carrying reference elements, for which thevalues on transmission are known to the receiver that receives them,this process being of the type comprising a step in which the phasevariation δ_(n) between at least two symbols carrying the referenceelements is estimated, in which the contribution of at least some of thesaid reference elements in the said estimate is weighted by informationrepresentative of the noise affecting the carrier frequency carrying thesaid reference element.

Thus, the importance assigned to each pilot depends on the disturbancesthat affect it, and therefore its credibility. It should be noted thatthis new approach is not obvious, but is based particularly on theformulation of the problem described above, that has never been made orenvisaged before.

Note that in general, the phase shift is induced particularly (but notexclusively) by the receiver.

The said information representative of the noise may in particularbelong to the group comprising:

the variance of the noise affecting each carrier frequency (σ² _(k));

the amplitude of the product R_(n,k).R*_(n−i,k);

the error rate affecting each carrier frequency;

information indicating that the carrier is not reliable.

This information, and particularly the noise variance, has often alreadybeen calculated for other applications, particularly the calculation ofthe signal to noise (S/N) ratio on each carrier to optimize Viterbidecoding with weighted inputs (“soft decoding”).

Therefore, very few calculations need to be carried out. All that isnecessary is to weight normal calculations as a function of availableinformation.

Thus, when estimating the variance, the estimate of the phase variationδ_(n) is advantageously calculated as follows: $\begin{matrix}{\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {\frac{1}{\sigma_{k}^{2}}\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}} & (3)\end{matrix}$

where P_(n,k) is the value of the reference element as it was emitted;

R_(n,k) is the value of the reference element as it was received;

n is the time dimension;

k is the frequency dimension;

C is the set of carrier frequencies carrying reference elements.

When the receiver calculates an estimate of ρn,k=A_(n,k)/σ² _(k), whereσ² _(k) is an estimate of the noise variance and A_(n,k) is an estimateof the modulus of the square of the frequency response of the channel oncarrier frequency k at instant n, the estimate of the phase variationδ_(n) may for example be calculated as follows: $\begin{matrix}{\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {{\rho_{n,k}\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)}\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}} & (4)\end{matrix}$

According to one advantageous embodiment of the invention, the saidweighting consists simply of thresholding, and only reference elementscarried by carrier frequencies for which the said informationrepresentative of the noise exceeds a predetermined threshold are takeninto account.

It is verified that this solution, which is shorter but simpler, issufficient in most cases.

In this case, if the said information representative of the noise is anestimate of the noise variance, the estimate of the phase variationδ_(n) may be calculated as follows: $\begin{matrix}{\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {{I\left( \sigma_{k} \right)}\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}} & (5)\end{matrix}$

where: I(σ_(k)) is equal to 1 provided that σ_(k) is below the saidthreshold, and is otherwise equal to 0.

If the said information representative of the noise is an estimate ofρ_(n,k)=A_(n,k)/σ² _(k), the estimate of the phase variation δ_(n) isadvantageously calculated as follows: $\begin{matrix}{\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {J\quad \left( \rho_{n,k} \right)\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}} & (6)\end{matrix}$

where J(ρ_(n,k)) is equal to 1 provided that ρ_(k) is above the saidthreshold, and is otherwise equal to 0.

The invention also relates to receivers embodying this type of process.These receivers comprise estimating means that weight the contributionof each of the said reference elements in the said estimate byinformation representative of the noise affecting the carrier frequencycarrying the said reference element.

Preferably, the said information representative of the noise is anestimate of the noise variance affecting each carrier frequency (σ²_(k)).

Other characteristics and advantages of the invention will become clearafter reading the following description of a preferred embodiment of theinvention, given as a simple illustrative and non-limitative example ofthe single FIGURE that diagrammatically illustrates the processaccording to the invention.

As mentioned in the preamble, the invention is applicable to thereception of multi-carrier signals, and more precisely concerns theestimate of a phase shift induced mainly (but not exclusively) by thereceiver.

For example, the process according to the invention may be used tooptimize the precision of the phase shift used in documents in Frenchpatents FR-95 10067 and FR-95 10068 deposited by the applicants whodeposited this patent.

The invention also makes it very easy to improve the estimate of thephase shift between two consecutive or non-consecutive symbols, forexample to act on the automatic frequency control (AFC).

The single FIGURE illustrates the principle of the invention.

The signal transmitted through the transmission channel isconventionally received by reception means 1 that perform conventionaloperations in order to find the different transmitted carriers(amplification, filtering, FFT, etc.). Still conventionally, thereceived pilots R_(n,k) are extracted (2) from the received signal.

These values R_(n,k) are input to the calculation module 3 to estimatethe phase shift, which also receives known values P_(r,k) of the pilotsfrom an internal memory 4.

Furthermore, according to the invention, the estimating module 3receives the calculation of the variance σ² _(k) from noise variancecalculation means 5, and this variance will also be used by otherconventional applications. Several conventional techniques are known forcalculating σ² _(k).

In one variant of the invention, the variance σ² _(k) is replaced by thevalue ρ_(n,k)=A_(n,k)/σ² _(k). Other alternative or additionalinformation representative of interference sources can also be takeninto account.

The estimating module 3 outputs the estimate of the phase variationδ_(n) that may be used for various operations, and particularly tocontrol the AFC 6.

According to the invention, this information δ_(n) takes account of aweighting factor that depends on the confidence that can be assigned toeach carrier, due to detected interference sources. If module 5 outputsthe information ρ² _(k), the estimating module can then calculate:$\begin{matrix}{\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {\frac{1}{\sigma_{k}^{2}}\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}} & (3)\end{matrix}$

According to a simplified version, it may be sufficient to perform ashort and much simpler weighting: $\begin{matrix}{\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {{I\left( \sigma_{k} \right)}\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}} & (5)\end{matrix}$

where I (σ_(k)) is equal to 1 provided that σ_(k) is below a giventhreshold S, and is equal to 0 when σ_(k) is greater than S. In onephysical embodiment, the block that estimates σ_(k) outputs a controlsignal when σ_(k) exceeds S. The module 3 that calculates the sum (4)disables use of the current term when it receives this signal.

If the receiver calculates ρ_(n,k)=A_(n,k)/σ² _(k) instead of σ² _(k),the calculation becomes: $\begin{matrix}{\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {{\rho_{n,k}\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)}\quad \left( {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*} \right\rbrack} \right\}} \right.}} & (4)\end{matrix}$

In the simplified version, this gives: $\begin{matrix}{\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {J\quad \left( \rho_{n,k} \right)\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}} & (6)\end{matrix}$

where J(ρ_(n,k)) is equal to 1 provided that ρ_(n,k) is above athreshold S, and is equal to 0 when ρ_(n,k) is less than S. In onephysical embodiment, the block that estimates ρ_(n,k) outputs a controlsignal when ρ_(n,k) becomes less than S. The module 3 that calculatesthe sum (6) disables use of the current term when it receives thissignal.

What is claimed is:
 1. Process for estimating a parasite phase shift onreception of a multi-carrier signal formed by a time sequence of symbolsmodulating a plurality of carrier frequencies, at least some of saidcarrier frequencies for at least some of said symbols carrying referenceelements, for which the values on transmission are known to a receiverthat receives them, the process comprising a step of determining anestimate of a phase variation δ_(n) between at least two symbolscarrying said reference elements by analysis of said reference elements,wherein a contribution of at least some of said reference elements isweighted by information representative of a noise affecting said carrierfrequency carrying said reference element.
 2. Process according to claim1, wherein said information representative of said noise is a member ofthe group consisting of: a variance of said noise affecting each carrierfrequency (σ² _(k)); an amplitude of a product (R_(n,k)·R_(N−1,k)); anerror rate affecting each carrier frequency; external informationindicating that a carrier is not reliable.
 3. Process according to claim1, wherein said information representative of said noise is an estimateof said noise variance, and said estimate of said phase variation δ_(n)is calculated as follows:$\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {\frac{1}{\sigma_{k}^{2}}\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}$

where P_(n,k) is a value of said reference element as said referenceelement was emitted; R_(n,k) is a value of said reference element assaid reference element was received; n is a time dimension; k is afrequency dimension; C is a set of said carrier frequencies carryingsaid reference elements.
 4. Process according to claim 2, wherein saidinformation representative of said noise is an estimate ofρ_(n,k)=A_(n,k)/(σ² _(k)), where σ² _(k) is an estimate of said noisevariance and A_(n,k) is an estimate of a modulus of a square of afrequency response of a channel on carrier frequency k at instant n, andin that said estimate of said phase variation δ_(n) is calculated asfollows:$\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {{\rho_{n,k}\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)}\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}$

where P_(n,k) is a value of said reference element as said referenceelement was emitted; R_(n,k) is a value of said reference element assaid reference element was received; n is a time dimension; k is afrequency dimension; C is a set of said carrier frequencies carryingsaid reference elements.
 5. Process according to claim 1, wherein saidweighting comprises thresholding, and only reference elements carried bycarrier frequencies for which said information representative of saidnoise exceeds a predetermined threshold are taken into account. 6.Process according to claim 5, wherein said information representative ofsaid noise is an estimate of said noise variance, and in that saidestimate of said phase variation δ_(n) is calculated as follows:$\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {{I\left( \sigma_{k} \right)}\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}$

where: I(σ_(k)) is equal to one provided that σ_(k) is below saidthreshold, and is otherwise equal to 0; where P_(n,k) is a value of saidreference element as said reference element was emitted; R_(n,k) is avalue of said reference element as said reference element was received;n is a time dimension; k is a frequency dimension; C is a set of saidcarrier frequencies carrying said reference elements.
 7. Processaccording to claim 5, wherein said information representative of saidnoise is an estimate of ρ_(n,k)=A_(n,k)/(σ² _(k)), where σ² _(k) is anestimate of said noise variance and A_(n,k) is an estimate of a modulusof a square of a frequency response of a channel on carrier frequency kat instant n, and in that said estimate of said phase variation δ_(n) iscalculated as follows:$\delta_{n} = {{Arg}\left\{ {\sum\limits_{k \in C}\left\lbrack {J\quad \left( \rho_{n,k} \right)\left( {R_{n,k} \cdot P_{n,k}^{*}} \right)\quad {\left( {R_{{n - 1},k} \cdot P_{{n - 1},k}^{*}} \right)\quad}^{*}} \right\rbrack} \right\}}$

where: J(ρ_(n,k)) is equal to one provided that ρ_(n,k) is below saidthreshold, and is otherwise equal to 0; where P_(n,k) is a value of saidreference element as said reference element was emitted; R_(n,k) is avalue of said reference element as said reference element was received,n is a time dimension; k is a frequency dimension; C is a set of saidcarrier frequencies carrying said reference elements.
 8. Receiver of amulti-carrier signal formed by a time sequence of signals modulating aplurality of carrier frequencies, at least some of said carrierfrequencies for at least some of said symbols carrying referenceelements, for which values on transmission are known to a receiver,comprising means for estimating a phase variation δ_(n) between at leasttwo symbols carrying reference elements, by analysis of said referenceelements, wherein said estimating means weight a contribution of each ofsaid reference elements in said estimate by information representativeof a noise affecting a carrier frequency carrying said referenceelement.
 9. Receiver according to claim 8, wherein said informationrepresentative of said noise is an estimate of a noise varianceaffecting each carrier frequency (σ² _(k)).